This invention relates to chromatic dispersion compensation, but more particularly to compensation of chromatic dispersion, which is performed post detection in the electrical domain.
With the advent of optical amplifiers, which can compensate for fibre loss, the reach of fiber systems, especially at 10 Gb/s and beyond, is limited by chromatic dispersion. Chromatic dispersion causes different parts of the signal spectrum to arrive at the distant end of the system at different times. An optical signal carrying information has a bandwidth spread related to the modulation of the optical carrier.
In the time domain, the dispersion can cause pulses to interfere with each other. This is known as inter-symbol interference or ISI. The dominant cause of chromatic dispersion is material dispersion, the variation in the refractive index versus wavelength of silica, the basic material from which all low loss transmission fibers are made. Ideally, chromatic dispersion is a reversible process. Optical dispersion compensation requires an element, which can produce a delay versus frequency characteristic equal and opposite to that of the fiber. It may be optically compensated either by using special dispersion shifted transmission fiber in the transmission path or by localized dispersion compensation. In the case of the dispersion shifted transmission fiber, a length of this highly dispersive fiber is inserted between the end of the channel and a PIN diode detector to add a frequency dependent delay opposite to that applied by the fiber in the channel. Unfortunately, the problem with this method is that it also introduces attenuation, it requires complex measurements and trained personnel for installation and is not adjustable once installed. Other methods include stretchable chirped Fiber Bragg Gratings (FBGs), Arrayed Waveguide Gratings (AWGs) and tunable Fabry Perot interferometers. Unfortunately, these methods have limited bandwidths, demand higher power levels, have slow and limited adaptation and are expensive.
Although efforts have been made to develop fully optical networks including photonic switching devices, the effects caused by chromatic dispersion and the solutions used to date have hampered their introduction since compensating devices have to be tailored to the specific length of fiber channel being used. Any changes due to network re-configuration or other changes require additional measurements and re-installation of chromatic dispersion compensating equipment.
A need therefore exists for a chromatic dispersion compensation process, which overcomes the shortcomings associated with the current compensation methods.
According to a first aspect of the invention, there is provided a method and apparatus for compensating, in the electrical domain, for chromatic dispersion of an optical signal
An optical signal is converted to an electrical signal comprised of components which are recoverable and components which are not directly recoverable. A first approximation of the output is derived by applying a transfer function to the electrical signal. The components of the electrical signal which are not directly recoverable are then estimated by applying a second transfer function to the first approximation. After removing these components a second approximation of the output is obtained by re-applying the first transfer function to the result. A better approximation may be achieved by re-applying the above method iteratively. According to another aspect of the invention,the optical signal may have a suitably small extinction ratio to improve compensation. The square root of the electrical signal may be taken to improve compensation.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.